How do I select a given number of rows for one table for each parent primary key in another table in sql server 2012?

Question

I have two tables in sql. One is a table of test cases and the other is a table of test runs with a foreign key that links back to a test case. I want to get the most recent 10 test runs for each test case. I don't want to loop through if I don't have to, but I don't see any other way to solve this problem. What is the most effective way to handle this sort of thing in sql server?

Answer 1

The idea:

select
    ...
from <test cases> as tc
    outer apply (
        select top 10 *
        from <test runs> as tr
        where
            tr.<test case id> = tc.<id>
        order by tr.<date time> desc
    ) as tr

or, if you just need to get data from table:

;with cte_test_runs as (
   select
        *,
        row-Number() over(partition by <test case id> order by <date time> desc) as rn
    from <test runs>
)
select *
from cte_test_runs
where rn <= 10

Answer 2

You can use Row No. Use Inner of Left Join as the case may be..

Select * from testCase a 
left outer join 
(Select Row_number() over (partition by testcase order by RecentDate desc ) RowNo, *  from TestRuns) b
on a.pk = b.fk
where b.RowNo <=10

Answer 3

You could use CROSS APPLY to select the top 10 per testCase ordered by date (or anyother ordering criteria).

See the related question 'When should I use Cross Apply over Inner Join?' which explains it better than I can. https://stackoverflow.com/a/1139231/1620715

In your case:

SELECT * from TestCase T
CROSS APPLY 
( SELECT TOP 10 * from TestRuns R where T.TestCaseId = R.TestCaseId
 order by R.TestDate desc) TopResults